Research On Testing Method Of Error Characteristics And Structure Optimization Design For Three-phase Three-element Combined Transformer

As an important component of power system,the distribution network system features a wide range and a large sale,it is one of key technologies to test precisely and reduce the tra nsformer?error in the distributionnetwork in the field of electric energy metering.Three-phase three-element combined transformers are widely used in distribution network,and traditional testing methods of its error conclude single-phase testing method and indirect testing method.The results of single-phase testing method couldn?t accurately reflect the true error characteristics of three-phase three-element combined transformer because of without considering the influence of the electromagnetic field on the transformer error.The results gained according to indirect testing method are the maximum range of transformer error,but not the true error of three-phase three-element combined transformer,and there are disadvantages such as high testing difficulty and poor data reliability.The real error characteritics of three-phase three-element combined transformer could not be detected with presented testing method,which results in the feature of three-phase three-element combined transformer in operation unknown.Consequently,in order to ensure its good metering performance under operational condition and the fair of power trade,it is of great academic significance and engineering value to study the testing method and performance improvement of three-phase three-element combined transformer?error.In this paper,three-phase three-element combined transformer with three-column core was odapted as research object.Firstly,the problems of testing the three-phase three-element combined transformer?error according to single-phase testing method and indirect testing method separately were analyzed.Secondly,based on the analysis of three-phase testing method?s pricinple,a high-voltage standard current transformer whose primary voltage was 10 kV was developed,and an error testing platform of three-phase three-element combined transformer used for three-phase testing method which was based on standard transformer was established.After that,error characteristics of three-phase three-element combined transformer were tested and analyzed under different loads.Finally,aimed at the disadvantage of three-phase three-element combined transformer with three-column core,a new design of three-phase three-element combined transformer with three-columnr core was proposed,and its error characteristics were analyzed with simulation.The main research achievements of this paper are as following:(1)It was discovered that the resuts of testing the error of three-phase three-element combined transformer according to typical testing method couldn't reflect its ture error characteristics,and three-phase testing method should be adopted to tset its error.Researches showed that the voltage error of three-phase three-element combined transformer tested according to single-phase testing method was the error of two windings in series,and the voltage of each winding was not primary voltage and the load of each winding was about 2 times of that applied,more over,single-phase testing method neglected the effect of current on voltage error and the effect of voltage on current error.The testing results according to indirect testing method were always higher than the actual values,which leaded to contradiction whether the transformer was qualified evaluated by testing results of single-phase testing method and indirect testing method.The operational conditions of three-phase three-element combined transformer could be simulated using three-phase detection,and testing results could reflect the error characteristics of three-phase three-element combined transformer precisely,so it is more reasonable and effective to test error characteristics of three-phase three-element combined transformer odopting three-phase detection.(2)A high-voltage standard current transformer whose rated voltage is 10 kV was successfully developed and used for three-phase testing method,and an error testing platform of three-phase three-element combined transformer used for three-phase testing method which was based on standard transformer was established.A shielding layer was added between primary winding and secondary winding and core respectively of the high-voltage standard current transformer,and the shielding layer was grounded through compensation winding,which solved the difficult problem of the influence of eakage current on standard current transformer,and the high-voltage standard current transformer met the requirement of error limit with 0.05 S.An error testing platform of three-phase three-element combined transformer used for three-phase testing method which was based on standard transformer was established,and met the requirement of testing the error of three-phase three-element combined transformer with three-phase testing method.(3)The error characteristics of three-phase three-element combined transformer under different loads were discovered based on three-phase testing method.Researches showed leakage currents would shift the error of current transformer to negative values,and its influence was mainly reflected in low current.Voltage error of three-phase three-element combined transformer changed linearly with current,and the differences of voltage errors of each phase was relatively large,and the algebraic sum of error variation of two phases was nearly equal the error variation of the third phase.Voltage error of three-phase three-element combined transformer would be influenced under primary load imbalance separately,but it not the case for current error.Current error wouldn?t be influenced under secondary burden imbalance of current transformer,but voltage error would change linearly with secondary burden imbalance of voltage transformer.(4)A new design of three-phase three-element combined transformer with three-columnr core was proposed,and the magnetic field distribution of voltage transformer?core and error characteristic of voltage transformer of three-phase three-element combined transformer were obtained.Aimed at the disadvantage of incomplete symmetry of three-phase three-element combined transformer with three-column core,a new structurally optimizing scheme of three-phase three-element combined transformer with three-columnr core was proposed.The new transformer has two merits,the first is the core of voltage transformer is three-columnar with the feature of complete symmetry and stereoscopic structure,and the second is that the three current transformers are equilateral triangle arrangement and the location layout of current transformers and voltage transformers is symmetric.Through simulating analysis of magnetic field of voltage transformers?core and error characteristic of voltage transformer of three-phase three-element combined transformer with three-columnr core,it was discovered that magnetic field distribution of voltage transformers?core of new transformer was more uniform,and error characteristic was better.